The present invention relates to a semiconductor device with thin film transistors (TFT) or the like, which are formed on a glass substrate and which employ crystalline semiconductor films, and to the method for fabricating the semiconductor device. The semiconductor device of the present invention includes liquid crystal display devices, EL display devices, EC display devices, image sensors, and the like, which have elements such as thin film transistors (TFT) and MOS transistors as well as semiconductor circuits (mircroprocessors, signal processing circuits, or high frequency circuits) comprising these insulated gate transistors. In addition, the semiconductor device of the present invention also includes electronic apparatuses such as video cameras, digital cameras, projectors, goggle displays, car navigation devices, personal computers, and portable information terminals in which these display devices are mounted.
Currently, thin film transistors (TFT) have been frequently used as semiconductor elements that employ semiconductor films. TFTs are used for a variety of integrated circuits, in particular, as switching elements of pixel portions of an active matrix liquid-crystal display device. Moreover, the mobility of the TFT has been improved recently to allow the TFT to be used as an element of the driver circuit for driving pixel portions. For a semiconductor layer for use in driver circuits, it is necessary to employ crystalline semiconductor films that are higher in mobility than amorphous semiconductor films. This crystalline semiconductor film is also called, for example, polycrystalline semiconductor film, polysilicon film, microcrystal semiconductor film and so forth.
What is crucial in improving the property of the TFT is to decrease impurities contained in the semiconductor layers thereof.
Furthermore, as the substrate for use in an active matrix type liquid crystal display device, there have been conventionally used transparent insulating substrates such as quartz substrates and glass substrates. The quartz substrate has a high glass distortion temperature and thus can be increased in crystallization temperature up to the order of 1000xc2x0 C. so as to decrease the time required for crystallization. However, the quartz substrate is very expensive compared with the glass substrate. Thus, from the point of view of mass production, it is difficult to employ it for large area use. Accordingly, as the substrate for use in a liquid crystal display device, a glass substrate such as Coming 7059 glass with a glass distortion temperature of 593xc2x0 C. has been widely employed.
The glass substrate for use in the liquid crystal display device has a lower content of impurities such as sodium (Na) than typical soda lime glass. However, it causes a problem to arise wherein a slight amount of impurities such as sodium (Na) diffuses at the time of heating (annealing) the substrate in a process such as in a crystallization process of amorphous semiconductor films to contaminate the semiconductor layers, which become active layers of the TFT, to deteriorate the TFT properties.
In general, in order to prevent the active layers from being contaminated due to sodium coming from the glass substrate, there is provided a silicon nitride film or silicon oxide film as a blocking film between the glass substrate and the active layers. In particular, the silicon nitride film is frequently used because of its high blocking performance to prevent the diffusion of sodium. However, the silicon nitride film provided on the glass substrate is given a significant stress. This leads to a problem in that an annealing process causes the silicon nitride film or films in contact therewith to crack, or the glass substrate to become deformed, damaged and so on. Accordingly, attempts have been made to use a multilayer film of a silicon nitride film and silicon oxide film, or silicon nitride oxide film.
However, the aforementioned multilayer film or the silicon nitride oxide film used as the blocking film had lower blocking performance than that of a simple silicon nitride film and thus could not ensure the prevention of contaminating impurities from coming out of the substrate. This is caused by the fact that, for the multilayer film, the thickness of the silicon nitride film cannot be made thicker to prevent cracks or the like; for the silicon nitride oxide film, the percentage of nitrogen content cannot be increased to prevent cracks or the like. On the other hand, the glass substrate contains a large amount of sodium, and thus a blocking film has been required which has a high blocking performance.
Furthermore, in the case of a bottom gate TFT, the blocking film is provided as an underlying film that is in contact with the bottom surface of the gate wiring (in this specification, it is to be understood that the gate wiring includes the gate electrode as well). In the case of employing silicon nitride film as the blocking film, this caused peeling to occur in the film of the gate wiring provided on the silicon nitride film, presenting a problem of bad adhesion of the silicon nitride film to the gate wiring.
In view of the aforementioned problems, in a semiconductor device with crystalline semiconductor layers crystallized in an annealing process, the object of the present invention is to form a semiconductor device such as an active matrix type liquid crystal display device at low cost with a large display area and high performance, which is provided with a blocking film that ensures the prevention of diffusion impurities such as sodium to improve TFT properties and which prevents cracks or the like in the coating from decreasing the yield. Moreover, in the bottom gate TFT, another object is to form a blocking film with an excellent adhesion to the gate wiring.
In order to solve the aforementioned problems, the present invention is characterized in that a tantalum oxide (TaOx) film is provided on a glass substrate as a blocking film. In the present invention, the tantalum oxide film refers to a coating predominantly composed of tantalum and oxygen. The tantalum oxide film can be used in thickness within the range of 100 through 500 nm. As for the method for forming the film, the thermal CVD method, the plasma CVD method, a deposition method, a sputtering method, the low pressure thermal CVD method, the thermal oxidation method, the anodizing method or the like can be employed. The tantalum oxide film can effectively block impurities such as sodium. In particular, the sputtering method or the plasma CVD method allows for controlling the stress in the tantalum oxide film to be formed by the deposition conditions, thereby providing an effective means for improving the blocking performance as well as reducing the stress caused by the glass substrate. Among them, in particular, it is most effective to employ the sputtering method which employs a target composed of tantalum and is carried out an atmosphere containing an oxygen gas mixture.
Still furthermore, the tantalum oxide film has excellent adhesion to a conductive film made of a material predominantly composed of one or more elements selected from the group consisting of tantalum (Ta), tungsten (W), molybdenum (Mo), titanium (Ti), chromium (Cr), and silicon (Si) which is typically used as the gate wiring, or to a conductive film made of a material with a melting point equal to those of these elements or higher. Thus, in the bottom gate TFT, this allows for preventing a decrease in the yield due to mal-adhesion of the blocking film to the gate wiring.
As the configuration of the present invention, the contamination derived from the glass substrate to the active layer due to impurities can be prevented by means of the tantalum oxide film. Thus, the concentration of sodium in the gate insulating film, which is provided by the secondary ion mass spectrometry (hereinafter referred to as SIMS), can be made equal to 1xc3x971016 atoms/cm3 or less which is currently equal to or less than the lower limit of detection in consideration of noise. This allows for making the concentration of sodium in the active layer to contact with the gate insulating film equal to 1xc3x971016 atoms/cm3 or less, thereby providing improved reliability of TFTs. Moreover, use of the tantalum oxide film as the blocking film can contribute to reducing variations in TFT properties, thereby providing improved reliability of TFTs.
Furthermore, in the configuration of the present invention, it is also effective to provide the substrate, not on only one side thereof but on both sides thereof, with tantalum oxide films. The tantalum oxide films provided on both sides of the substrate can completely block impurities such as sodium diffusing from the substrate at the time of fabricating the semiconductor device. In addition, the glass substrate used in a liquid crystal display device is slightly etched in an etching process that employs a specific acid solution such as a fluoric acid to allow impurities such as sodium (Na) in the substrate to be mixed into the acid solution, leading to a problem in that semiconductor layers that are to serve as the active layers of TFTs are contaminated to cause the TFT properties to deteriorate. Because the tantalum oxide film provides excellent resistance to most acid solutions such as fluoric acid, it is possible to prevent sodium contamination in the etching process by providing the tantalum oxide films on both sides of the substrate. In addition, because of a small coefficient of thermal expansion and outstanding resistance to heat of the tantalum oxide film, the heat resistance of the substrate can be improved by providing the tantalum oxide film on the reverse side of the substrate. Still furthermore, it is also effective to cover (coat) the entire surface of the substrate, including the side surfaces of the substrate, with the tantalum oxide film. The low pressure thermal CVD method can be used to facilitate forming a coating on the side surfaces of the substrate.
A first configuration of the present invention to be disclosed in this specification is characterized by comprising:
a blocking film in contact with a glass substrate,
a gate wiring in contact with said blocking film,
a gate insulating film in contact with said gate wiring, and
a crystalline semiconductor layer, in contact with said gate insulating film, comprised of a high concentration impurity region, a channel formed region, and a low concentration impurity region between said high concentration impurity region and said channel formed region,
wherein said blocking film is made of tantalum oxide.
Furthermore, the structure is characterized in that said gate insulating film has a concentration of sodium of 1xc3x971016 atoms/cm3 or less.
A second configuration of the present invention to be disclosed in this specification is characterized by comprising the steps of:
forming a tantalum oxide film on a glass substrate,
forming a gate wiring on said tantalum oxide film,
forming a multilayer of a gate insulating film and a semiconductor film on the glass substrate with said gate wiring formed thereon,
crystallizing said semiconductor film into a crystalline semiconductor film,
patterning said crystalline semiconductor film to form a crystalline semiconductor layer,
adding selectively an impurity element selected from the group consisting of group XIII and XV into said crystalline semiconductor layer to form a high concentration impurity region, and
adding selectively an impurity element selected from the group consisting of group XIII and XV into said crystalline semiconductor layer to form a low concentration impurity region.
Furthermore, a third configuration of the present invention to be disclosed in this specification is characterized by comprising the steps of:
forming a tantalum oxide film on a glass substrate,
forming a gate wiring on said tantalum oxide film,
forming a multilayer of a gate insulating film and a semiconductor film on the glass substrate with said gate wiring formed thereon,
adding an element into said semiconductor film to accelerate crystallization of the semiconductor film,
crystallizing said semiconductor film into a crystalline semiconductor film,
removing the element, in said crystalline semiconductor film, accelerating said crystallization,
patterning said crystalline semiconductor film to form a crystalline semiconductor layer,
adding selectively an impurity element selected from the group consisting of group XIII and XV into said crystalline semiconductor layer to form a high concentration impurity region, and
adding selectively an impurity element selected from the group consisting of group XIII and XV into said crystalline semiconductor layer to form a low concentration impurity region.
Still furthermore, the configuration is characterized in that at least one element selected from the group consisting of Ni, Co, Fe, Pd, Pt, Cu, Au, Ge, Sn, and Pb is used as an element for accelerating said crystallization.